The invention relates to a transmit and receive antenna on board a satellite forming part of a telecommunications system in which said antenna relays calls in a terrestrial region divided into a plurality of zones. The region is divided into zones by allocating to each zone a primary source consisting of individual radiating entities that can be common to a plurality of sources.
Compared to global coverage, dividing the region covered by the satellite into zones has the advantage that energy performance is improved and frequencies can be re-used from one zone to another. For example, the allocated frequency band can be divided into a plurality of sub-bands and the sub-bands can be distributed so that two adjacent zones use different sub-bands.
A region covered by a satellite is divided into zones both for geosynchronous satellites and for non-geosynchronous satellites. The following description is limited to a geosynchronous satellite telecommunications system, but the invention also applies to a non-geosynchronous satellite system for communicating with mobiles.
The example mainly considered will be that of a Ka band telecommunications system for high bit rate multimedia services. In the Ka band, the transmit frequency is 20 GHz and the receive frequency is 30 GHz. These high frequency values enable the use of relatively compact equipment both on board the satellite and on the ground, and therefore reduce costs, which in the case of the terrestrial equipment is beneficial from the point of view of mass production.
A typical geosynchronous satellite telecommunications system covers a region xe2x80x9cseenxe2x80x9d by the satellite within a total angle of approximately 6xc2x0, and the region is divided into about 40 to about 100 zones. In this system, each zone is formed by a linearly (or circularly) polarized beam which is highly directional, having a directionality of the order of 45 dBi at the edge of the coverage zone, the frequency band is divided into four sub-bands, and the secondary lobes of each beam must have a low level relative to the main lobe in order to limit interaction between zones using the same frequency. It is generally accepted that the level of the secondary lobes must be at least 25 dB below the level of the main lobe.
The large number of zones for the same region leads to a large number of primary sources, which is not beneficial in terms of minimizing the mass and the volume of the equipment on board the satellite.
The equipment on board the satellite includes reflectors, each of which is associated with a plurality of primary sources, and each source corresponds to a terrestrial zone, but is able to contribute to the generation of several zones. Thus FIG. 1 is a diagram showing a reflector 10 in whose focal plane 12 there are a plurality of primary sources, only two of which are shown, namely the sources 14 and 16. The source 14 transmits or receives a beam whose edge rays are denoted 141 and 142 in FIG. 1. The primary source 16 transmits or receives a beam whose edge rays are denoted 161 and 162. Each of the beams 141, 142 and 161, 162 forms a terrestrial zone with a diameter of at least 100 kilometers. The diameter of the reflector 10 is of the order of 1 meter or 1.5 meters and it is therefore sufficient for each beam to have an aperture of a few tenths of a degree to obtain the corresponding relationship between the primary source, the reflector and the terrestrial zone, for transmission in particular.
Because each primary source 14, 16 is of non-negligible overall size, each reflector 10 is associated with primary sources corresponding to distant zones. The greater the distance between the terrestrial zones, the greater the distance required between the primary sources 14, 16, also referred to as the pitch. Accordingly, as a general rule, the primary sources associated with two adjacent zones are allocated to different reflectors. In one example, one-fourth of the primary transmit and/or receive sources are allocated to each reflector.
It is therefore clear from the FIG. 1 diagram that the distance on the ground between terrestrial zones conditions the distance between radiating sources 14, 16 and that the dimension of each terrestrial zone conditions the diameter of the reflector 10.
The combination of the reflector and the radiating sources must satisfy two additional conditions relating to the illumination of the reflector by a primary source, over and above the conditions referred to above relating to secondary lobes:
The first condition is that the source must illuminate the periphery 20 of the reflector 10 at a sufficiently low level for the radiation not to interfere with the terrestrial zones adjoining the area to which that source is allocated.
The second condition is that the primary source must illuminate the periphery 20 of the reflector 10 at a sufficiently high level to guarantee good surface efficiency (the ratio between the actual directionality of the beam and the maximum directionality of the antenna for uniform illumination).
For example, the peripheral zone 20 must be illuminated at a level approximately 9 dB below the level of the illumination of the central zone 22 to obtain a good trade-off between these two contradictory constraints.
Finally, for each chosen circular zone to be illuminated optimally, the radiation pattern of each primary source must also be circularly symmetrical, both for transmission and for reception.
Because the radiation pattern of a source is frequency-dependent, it is different for transmission and for reception. Consequently, to comply easily with the conditions imposed on the radiating source and reflector combination as a whole, it is preferable to separate the sources provided for transmission from the sources provided for reception.
Accordingly, a routine radiating source and reflector combination includes first reflectors for the transmit sources and second reflectors for the receive sources. Although that solution complies with the constraints regarding isolation between zones and efficiency for each beam, it nevertheless has the disadvantage of leading to large overall size and high mass for the equipment on board the satellite. Also, the large number of reflectors increases the complexity of the mechanical assembly on board the satellite.
The number of reflectors on a satellite can be reduced by using the same radiating source to transmit and receive. This is known in the art.
To this end it is necessary to use wide-band sources (i.e. sources operating both in the transmit band and in the receive band). In this case, the choice of the source is in practice limited to a xe2x80x9ccorrugatedxe2x80x9d radiating aperture, i.e. one having internal ribs, because that type of source is the only one that can produce a circularly symmetrical pattern for the transmit and receive frequencies with a satisfactory reflection coefficient, also referred to as the standing wave ratio (SWR).
However, for a given directionality, a corrugated radiating aperture is of larger overall size than a narrow-band primary source (for example a Potter radiating aperture). This being the case, for a given distance between terrestrial zones allocated to the same reflector 10, a greater distance between primary sources is required, compared to the first embodiment.
Accordingly, in the FIG. 1 diagram, the sources 14 and 16 correspond to transmit (or receive) sources in the first embodiment described and the overall sizes of the transmit and receive sources 14xe2x80x2 and 16xe2x80x2 are increased. It can therefore be seen that in the second embodiment, because the distance between the sources is greater, the positioning of the areas on the ground no longer complies with the imposed constraints. The size of the corrugated radiating apertures must therefore be reduced, which leads to excessive illumination of the periphery 20 of the reflector 10 (generally only 3 dB below the illumination at the center 22). This excessive illumination interferes with the operation of the system and leads to energy losses.
The invention aims to provide a transmit and receive system in which each wide-band primary source is free of the disadvantages of the prior art solutions, i.e. achieves a sufficiently low level of illumination at the periphery of the transmit reflector.
Thus in an antenna of the invention each reflector is associated with a plurality of transmit and receive sources and each transmit and receive source includes a plurality of radiating apertures whose efficiency (gain) is at least equal to 70%, with individual feed means for feeding each radiating aperture able to supply different energies to two different radiating apertures in order for the illumination at the periphery of the reflector to be at a sufficiently low level for the energy radiated outside the reflector to be negligible, and preferably so that the illumination at the periphery is practically the same for all transmit and receive frequencies.
Other things being equal, and in particular the area of the reflector, for example that of a circle with a diameter of approximately 50 mm, compared to a corrugated radiating aperture, each radiating aperture, which has an efficiency of at least 70%, is more directional, which reduces the energy at the edge of the reflector. A corrugated radiating aperture has an efficiency (gain) of at most 60%.
It should be noted that, until now, it has been considered that a high-efficiency smooth conical horn radiating aperture is not suitable for this type of wide-band source because it cannot produce a circularly symmetrical radiation pattern and the radiation pattern has large secondary lobes, preventing correct isolation between zones to which the same frequency sub-bands are allocated. However, the invention overcomes at least the major part of this drawback, because the radiating sources are not very directional, compared to the source consisting of the set of apertures, and the distribution of the radiation from each high-efficiency radiating aperture reduces the overall lack of symmetry about the axis of the reflector, because it reduces the difference between the radiating levels in two planes perpendicular to each other and to the reflector.
A high-efficiency central radiating aperture and high-efficiency peripheral radiating apertures are preferably distributed regularly about the axis of the central radiating aperture, for example. In one embodiment, the power fed to a high-efficiency central radiating aperture is greater than the power fed to the high-efficiency peripheral radiating apertures and the peripheral radiating apertures are all fed with the same power.
Generally speaking, the invention provides a feed for each radiating aperture and the amplitude and the phase of each feed can be chosen at will for transmission and for reception. In other words the radiation pattern in transmission and in reception can be selected at will, thanks to the multiplicity of radiating apertures and the individual feed to each radiating aperture.
Thus it will often be of benefit to feed the radiating apertures differently for transmission and for reception.
To improve the symmetry of the radiation pattern about the axis of the reflector, or about the axis of the set of radiating apertures, according to one feature of the invention, the various radiating apertures are fed with linear polarization and the polarization is oriented relative to the disposition of the various radiating apertures to maximize the symmetry of the radiation about the axis of the radiating source. For example, if the radiating apertures are distributed so that there is a direction passing through the center of the radiating source through which a maximum number of centers of radiating apertures passes, the polarization direction perpendicular to that direction is chosen.
To prevent the lobes of the array of radiating apertures constituting the radiating source reducing the power to be transmitted in the wanted direction, the distance between the centers of the radiating apertures is less than one wavelength at the transmit frequency (the lower frequency). For example, when the transmit frequency is 20 GHz, the distance between the radiating apertures must be less than approximately 16 mm.
The present invention provides a radiating source for transmitting and receiving at different frequencies, intended to be installed on board a satellite to define a radiation pattern in a terrestrial zone, said source being intended to be disposed in or near the focal plane of a reflector associated with other sources corresponding to other terrestrial zones, the source including a plurality of radiating apertures, each of which has an efficiency at least equal to 70%, and feed means for feeding each radiating aperture, the radiating apertures and their feed means being such that the energy radiated by all of the radiating apertures is practically limited to the corresponding reflector, at least for transmission.
In an embodiment, the feed means of each radiating aperture are such that the radiation pattern is substantially the same for transmission and for reception.
In an embodiment, the source includes a central radiating aperture and peripheral radiating apertures.
In an embodiment, the peripheral radiating apertures are regularly distributed around the axis of the central radiating aperture.
In an embodiment, the feed to the central radiating aperture is such that said central radiating aperture produces the most radiation.
In an embodiment, the feed means for the peripheral radiating apertures are such that the radiation produced by each of said peripheral radiating apertures is of practically the same intensity and less than the intensity of the radiation produced by the central radiating aperture.
In an embodiment, the radiation to be transmitted by the source has linear polarization in a particular direction and the feed means are such that each radiating aperture transmits radiation polarized in said particular direction which is oriented relative to the set of radiating apertures in such a manner as to maximize the uniformity of the radiation in three dimensions.
In an embodiment, the polarization direction is chosen so that a straight line segment in that direction passing through the center of the exit plane of the source passes through a minimum number of radiating apertures.
In an embodiment, the radiating apertures and the feed means are such that the intensity of the transmitted radiation at the periphery of the reflector is approximately 9 dB below the intensity of the transmitted radiation in the central part of the associated reflector.
In an embodiment, the Ka band is used for transmission and for reception.
In an embodiment, the transmit frequency is of the order of 20 GHz and the receive frequency is of the order of 30 GHz.
In an embodiment, the distance between the axes of two adjoining radiating apertures is of the same order as the wavelength of the transmitted radiation.
The present invention also provides a telecommunications system in which calls are relayed by antennas on board a satellite, in particular a geosynchronous satellite, the system including an antenna with radiating sources each of which is a radiating source of the type defined hereinabove.